1. Field of the Invention
This invention relates generally to an optical disc player for reproducing optically an information signal from an optical disc, such as an optical digital audio disc, on which the information signal is recorded in a record track formed with an arrangement of a plurality of small pits, and more particularly is directed to a tracking control arrangement which is operative to maintain a light beam impinging on an optical disc to read an information signal therefrom in correct tracking relation to a record track on the optical disc in an optical disc player.
2. Description of the Prior Art
In an optical disc player for reproducing an information signal from an optical disc, such as an optical digital audio disc, on which the information signal is recorded in the form of small pits arranged in a spiral record track, a light beam is used for reading the information signal from the spiral record track on the optical disc. The light beam is emitted from an optical head which in moved in the direction of the radius of the optical disc, and is required to trace correctly the spiral record track on the optical disc. To make the light beam comply with this requirement, tracking control is performed. In the tracking control, the position of a beam spot formed by the light beam on the surface of the optical disc in relation to the spiral record track is detected to produce a tracking detection output, and a focusing lens in the optical head or the optical head in its entirety is then moved in the direction of the radius of the optical disc in response to the tracking detection output.
To detect the position of the beam spot on the surface of the optical disc in relation to the spiral record track, there have been proposed several detecting arrangements. These are classified into two types, one of which uses two special light beams provided in addition to the light beam for reading the information signal, and the other of which does not use any light beam other than the one for reading the information signal. As an arrangement of the type operative without two special light beams, a detecting arrangement such as so-called "heterodyne system" or "DPD system" has been known.
FIG. 1 of the accompanying drawings shows a previously proposed tracking control arrangement employing the detecting arrangement so called "heterodyne system" or "DPD system" in an optical disc player which has an optical head for causing a light beam to impinge on an optical disc having a spiral record track formed with pits thereon. In the optical head, a photodetector 10 composed of four photodetecting elements 10A, 10B, 10C and 10D is provided to receive the light beam modulated in intensity and reflected at the optical disc. The outputs of the photodetecting elements 10A to 10D are supplied to an operational circuit 20. In the operational circuit 20, the outputs of the photodetecting elements 10A and 10C are added to each other in an adding circuit 21, the outputs of the photodetecting elements 10B and 10D are added to each other in an adding circuit 22, the added outputs on the adding circuits 21 and 22 are further added to each other in an adding circuit 23 to produce an added signal RF.sub.0 which is the sum of the outputs of the photodetecting elements 10A to 10D, and subtraction of the added outputs of the adding circuits 21 and 22 is performed in a subtracting circuit 24 to produce a subtracted signal TE.sub.0 which is the difference between the added output obtained from the adding circuit 21 and the added output obtained from the adding circuit 22.
The light beam incident upon the optical disc for reading the information signal therefrom is diffracted by the pits arranged in the spiral record track on the optical disc to be reflected thereat. Accordingly, the reflected light beam modulated at the optical disc and reaching the photodetector 10 to form its beam spot on the photodetecting elements 10A to 10D forms a diffraction pattern varying in response to the positional relation between each pit on the optical disc and the beam spot on the optical disc formed by the light beam irradiating the pit.
FIGS. 2A, 2B and 2C of the accompanying drawings show such a diffraction pattern and the positional relation obtained in several different situations. In each of FIGS. 2A, 2B and 2C, the positional relation between the pit P and the beam spot S of the light beam irradiating the pit P is shown in the left side, and the diffraction pattern (a shaded portion) in the beam spot formed on the photodetecting elements 10A to 10D by the reflected light beam in consequence of the positional relation shown in the left side is shown in the right side. The pit P moves in relation to the beam spot S so that the situation shown in the upper side is changed into the situation shown in the lower side. In the case of FIG. 2A, the beam spot S is deviated inward on the optical disc from the center on the pit P. In the case of FIG. 2B, the beam spot S is correctly located at the center of the pit P. In the case of FIG. 2C, the beam spot S is deviated outward on the optical disc from the center of the pit P.
From the presentations of FIGS. 2A, 2B and 2C, it can be seen that the diffraction pattern which causes all the photodetecting elements 10A to 10D to be supplied with the same amount of light, is obtained, when the beam spot S is correctly located at the center of the pit P. The diffraction pattern becomes such that the amount of light supplied to the photodetecting elements 10A to 10D is asymmetric, when the beam spot S is deviated inward or outward on the optical disc from the center of the pit P. The form of asymmetry is opposite for deviation caused inward and outward, and the greater the deviation is, the greater the asymmetry occurs.
Consequently, the subtracted signal TE.sub.0 obtained from the subtracting circuit 24 in the operational circuit 20 as a result of the difference between the sum of the outputs of the photodetecting elements 10A and 10C and the sum of the outputs of the photodetecting elements 10B and 10D, can be used to produce a tracking error signal which represents the amount and direction of the deviation of the beam spot S from the center of the record track. The added signal RF.sub.0 obtained from the adding circuit 23 in the operational circuit 20 as a result of the total sum of the outputs of the photodetecting elements 10A to 10D, is used as a reproduced information signal.
In the case where the beam spot on the optical disc traces the record track formed with the arrangement of the pits P along a meandering path 1 as shown in FIG. 3A, the added signal RF.sub.0 used as the reproduced information signal is obtained in such a form as to have a falling cross-over point in respect of a constant level V.sub.0 when the beam spot passes the front edge of each pit P and to have a rising cross-over point in respect of the constant level V.sub.0 when the beam spot passes the rear edge of each pit P, as shown in FIG. 3B. On the other hand, the subtracted signal TE.sub.0 used to produce the tracking error signal is obtained in the respective different manners taken in the situation in which the beam spot is deviated inward on the optical disc from the center of the record track and the situation in which the beam spot is deviated outward on the optical disc from the center of the record track, respectively. In the situation in which the beam spot is deviated inward on the optical disc from the center of the record track as shown in FIG. 2A, the subtracted signal TE.sub.0 is positive when the beam spot passes the front edge of each pit P and therefore the added signal RF.sub.0 has the falling cross-over point in respect of the constant level V.sub.0, and is negative when the beam spot passes the rear edge of each pit P and therefore the added signal RF.sub.0 has the rising cross-over point in respect of the constant level V.sub.0, as shown in the left half of FIG. 3E. In the situation in which the beam spot is deviated outward on the optical disc from the center of the record track as shown in FIG. 2C, the subtracted signal TE.sub.0 is negative when the beam spot passes the front edge of each pit P and therefore the added signal RF.sub.0 has the falling cross-over point in respect of the constant level V.sub.0, and is positive when the beam spot passes the rear edge of each pit P and therefore the added signal RF.sub.0 has the rising cross-over point in respect of the constant level V.sub.0, as shown in the right half of FIG. 3E. The greater the deviation caused inward and outward is, the larger the amplitude of the subtracted signal TE.sub.0 becomes.
The added signal RF.sub.0 obtained from the operational circuit 20 is supplied to a voltage comparator 31. At the voltage comparator 31, the added signal RF.sub.0 is compared with the constant level V.sub.0 to produce a modified signal RF.sub.Z shaped into a rectangular waveform as shown in FIG. 3C. The modified signal RF.sub.Z is supplied to pulse generating circuits 32 and 33. From the pulse generating circuit 32, a pulse SP.sub.A having narrow pulse-width is obtained at each rising edge of the modified signal RF.sub.Z, and from the pulse generating circuit 33, a pulse SP.sub.B having narrow pulse-width is obtained at each falling edge of the modified signal RF.sub.Z, as shown in FIG. 3C.
The subtracted signal TE.sub.0 from the operational circuit 20 is supplied to switches 41 and 42 provided for sampling in sampling-and-hold circuits 40 and 50, respectively. The pulses SP.sub.A and SP.sub.B are also supplied to the switches 41 and 51, respectively, so that the level of the subtracted signal TE.sub.0 at the instant at which the beam spot on the optical disc passes the front edge of each pit P is sampled by the pulse SP.sub.A at the switch 41, and the level of the subtracted signal TE.sub.0 at the instant at which the beam spot on the optical disc passes the rear edge of each pit P is sampled by the pulse SP.sub.B at the switch 51. The level sampled at the switch 41 is held by a capacitor 42 provided for holding in the sampling-and-hold circuit 40, and the level sampled at the switch 51 is held by a capacitor 52 provided for holding in the sampling-and-hold circuit 50. Output signals TE.sub.A and TE.sub.B of the sampling-and-hold circuits 40 and 50, which are obtained across the capacitors 42 and 52 as shown in FIGS. 3F and 3G, respectively, are supplied to a differential amplifier 60 which performs subtraction of the output signals TE.sub.A and TE.sub.B to produce a signal TE.sub.C as shown in FIG. 3H at the output thereof. The signal TE.sub.C varies in its polarity, for example, from negative to positive when the beam spot moves to cross over the center of the record track outward on the optical disc, and from positive to negative when the beam spot moves to cross over the center of the record track inward on the optical disc. Further, the signal TE.sub.C has a level corresponding to the deviation of the beam spot on the optical disc from the center of the record track. Therefore, the signal TE.sub.C can be used as the tracking error signal which represents the amount and direction of the deviation of the beam spot on the optical disc from the center of the record track.
The signal TE.sub.C thus obtained is supplied to a driving circuit 70 provided for driving the focusing lens in the optical head or the optical head in its entirety to move in the direction of the radius of the optical disc, so that the position of the beam spot on the optical disc relative to the record track is controlled.
However, the previously proposed tracking control arrangement as described above encounters the following trouble when the optical disc has some non-reflecting portion due to damage or a stain on the surface thereof.
In the case where the optical disc has a non-reflecting portion 2 where the surface thereof is damaged or stained as shown in FIG. 3A, the added signal RF.sub.0 obtained from the operational circuit 20 takes a low level V.sub.L as shown by a dot-and-dash line in FIG. 3B which is out of a predetermined range of the amplitude of the added signal RF.sub.0 when the beam spot on the optical disc is formed on the non-reflecting portion 2, and accordingly the modified signal RF.sub.Z obtained from the voltage comparator 31 does not have the rectangular waveform corresponding to the arrangement of the pits P, as shown in FIG. 3E. As a result of this, the pulses SP.sub.A and SP.sub.B are not obtained from the pulse generating circuits 32 or 33, so that each of the output signals TE.sub.A and TE.sub.B of the sampling-and-hold circuits 40 and 50 is held at the level obtained just before the beam spot on the optical disc entered into the non-reflecting portion 2 while the beam spot is in the non-reflecting portion, as shown with a dot-and-dash line in FIG. 3F or FIG. 3G. Consequently, the signal TE.sub.C which is used as the tracking error signal and supplied to the driving circuit 70 is also held at the level obtained just before the beam spot entered into the non-reflecting portion 2 while the beam spot is in the non-reflecting portion 2 as shown with a dot-and-dash line in FIG. 3H and represents incorrectly the position of the beam spot on the optical disc in relation to the record track, so that track jump movement of the light beam by which the beam spot on the optical disc is undesirably moved rapidly in the direction transverse to the record tracks, is easy to be caused.